In photographic emulsions in general, it is often highly desirable to maximize the photosensitivity, or speed, of the emulsion. It is also often desirable, especially in graphic arts photographic products, such as halftone reproduction materials, to utilize photographic emulsions exhibiting high contrast.
In silver halide photographic emulsions, speed may be increased in a number of ways. Increasing the size of the silver halide grains increases photographic speed; however, it also contributes to a loss of image sharpness. Chemical sensitizers for silver halide, such as sulfur or gold compounds, can be included in the photographic emulsion to increase photographic speed, but the degree of speed increase is limited, and oversensitization can lead to fog or even to a decrease in speed. Spectral sensitizers, such as cyanine dyes, can also increase photographic speed, but the amount of speed increase can fall short of what is desired. Moreover, the above-described photographic speed-increasing techniques often do not increase contrast to the extent desired.